This invention relates to an improved switching apparatus for absolute length- or angle-measuring devices of the type which include a coded scale; a scanning unit for reading the coded scale, which scanning unit comprises a plurality of electrical components; means for generating a supply voltage across two voltage supply terminals; and means for generating a switching signal used to switch the electrical components of the scanning unit on and off.
In position measuring devices of the general type described above, it is standard practice to switch on the electrical components of the scanning unit only briefly during the interrogation of the scanning unit for absolute position values, in order to save current. It is a known practice to utilize a computer, included in the position measuring device, to control a switch which acts to switch the supply voltage supplying the scanning unit on and off. The switch has in the past been located outside of the scanning unit, for example in the computer itself. Furthermore, if the electronic components of the scanning unit, such as for example one or more illuminating elements such as light-emitting diodes, are connected over long conductors to the supply voltage, then it is standard practice to connect one or more blocking capacitors across the supply voltage in parallel with the electrical components. These blocking capacitors additionally prevent a possible ringing of the integrated circuits which are often included in the electrical components of the scanning unit.
When this prior art approach is used, each interrogation of the scanning unit to determine a position code requires that the switch be closed in order to charge these capacitors. In the case of relatively rapid switching, considerable charging currents can result. Furthermore, many elements of the scanning unit, such as light-emitting diodes, for example, cannot be activated until the blocking capacitors have been charged. In this way, the initiation of illumination and operation of the scanning unit can be delayed. Furthermore, once the switch is opened, the blocking capacitors may discharge through the electronic components of the scanning unit, thereby applying voltage to these elements for a longer period than is necessary. In this way, the period in which the light sources such as diodes are illuminated is extended beyond the desired period.